Power MOSFETs (Metal-Oxide-Semiconductor Field Effect Transistors) are commonly used in numerous applications, including power supplies, portable devices and automotive electronics. MOSFET is a type of three-terminal transistor having a gate, a source and a drain terminal. One of the tasks of the power MOSFETs in these applications is to provide switching function and control the power delivery from the source to the load. One of the most popular applications of the power MOSFETs is for switching multiple power sources in notebook computers. In this case, a common source configuration of two power MOSFETs, as shown in FIG. 1 is required. The two power MOSFETs are basically connected back to back with the sources connected together.
In a typical notebook power supply system, the AC adaptor voltage is always higher than that of the battery voltage. When the power MOSFET is off, current can still flow to the battery through the body diode, as shown in FIG. 2. To prevent this current flow, a true on/off switch is required. One of the solutions is to connect the two power MOSFETs in a common source configuration between the AC adaptor and the main battery as shown in FIG. 3. This design has been used commonly in the current notebook supply systems.
Current practice of the common source configuration is to connect the sources of the two discrete MOSFETs (housed either in separated packages or in a single package) externally on the printed circuit board as shown in FIG. 4. In the case of conventional dual MOSFETs in a single package, one gate pad per die is used for the gate interconnect. The gate posts, source posts, and drain posts are all separated from each other. For ease of wire bonding to the gate posts, the gate pad is generally located at the upper left corner. Because the gate pads and source pads of the two dies are alternatively placed between gate and source, it does not allow the sources of the power MOSFETs to be connected internally. This is because the alternating gate and source will cause the gate to be shorted to the source during wire bonding if the two sources are connected together internally. Further, an additional layer of circuit board is required to connect the sources externally. All of these approaches may be relatively costly, as the manufacturing of the devices may be more complicate.